Management of destage tasks with large number of ranks

ABSTRACT

A processor, operable in a computing storage environment, for each rank in a storage management device in the computing storage environment, allocates a lower maximum count defined by a predetermined lower maximum count of Task Control Blocks (TCBs) of a rank for performing destage operations, and a higher maximum count of TCBs to be implemented for performing a storage operation, and performs the storage operation using up to the lower maximum count of TCBs, yet only allows those TCBs above the lower maximum count to be allocated for performing the storage operation satisfying at least one criterion.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.14/074,076, filed Nov. 7, 2013, which is a Continuation of U.S. patentapplication Ser. No. 13/623,624, filed on Sep. 20, 2012.

FIELD OF THE INVENTION

The present invention relates in general computing systems, and moreparticularly to, systems and methods for increased data managementefficiency in computing storage environments.

DESCRIPTION OF THE RELATED ART

In today's society, computer systems are commonplace. Computer systemsmay be found in the workplace, at home, or at school. Computer systemsmay include data storage systems, or disk storage systems, to processand store data. Contemporary computer storage systems are known todestage, and subsequently, demote storage tracks from cache to long-termstorage devices so that there is sufficient room in the cache for datato be written.

SUMMARY OF THE INVENTION

A storage management device (e.g., storage controller) in a computerstorage system has a limited number of Task Control Blocks (TCBs). Inone such storage controller, the number of such TCBs may be 8000 (8K).In contrast the same storage controller may have a larger relativenumber of available ranks. In the instant example, the storagecontroller may have 400. An individual rank has a maximum number ofdestage TCBs (TCBs dedicated to executing destage operations in thestorage environment) running to destage modified data in Non VolatileStorage (NVS). In the present example, such maximum number of destageTCBs may be 40.

In view of such a scenario, if each rank is using its maximum number ofdestage TCBs, then the storage controller may run out of free TCBs(e.g., 40*400=16000>8K), or have very few destage TCBs to be used forother tasks in the storage controller (for example if many ranks arenear their maximum TCBs).

One possible solution is to cap the maximum destage TCBs for every rankto a lower number, such that every rank can have maximum destage TCBsand still the total will not exceed a threshold. This solution resultsin a scenario, however, where no rank may be using maximum destage TCBs,and the ranks that should use maximum destage TCBs will be running withfewer destage TCBs, and hence the throughput on the ranks, and overallsystem performance may suffer. A need exists to better manage destagetasks in a storage management device having a large number of ranks.

Accordingly, and in view of the foregoing, various embodiments for datamanagement in a computing storage environment are provided. In oneembodiment, by way of example only, a method of data management by aprocessor device in a computing storage environment is provided. Foreach rank in a storage management device in the computing storageenvironment, a lower maximum count defined by a predetermined lowermaximum count of Task Control Blocks (TCBs) of a rank for performingdestage operations, and a higher maximum count of TCBs to be implementedfor performing a storage operation are allocated, and the storageoperation is performed using up to the lower maximum count of TCBs, yetonly allows those TCBs above the lower maximum count to be allocated forperforming the storage operation satisfying at least one criterion.

Other system and computer program product embodiments are provided andsupply related advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages of the invention will be readilyunderstood, a more particular description of the invention brieflydescribed above will be rendered by reference to specific embodimentsthat are illustrated in the appended drawings. Understanding that thesedrawings depict only typical embodiments of the invention and are nottherefore to be considered to be limiting of its scope, the inventionwill be described and explained with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIG. 1 is an exemplary block diagram showing a hardware structure forcache management in which aspects of the present invention may berealized;

FIG. 2 is an exemplary block diagram showing a hardware structure of adata storage system in a computer system according to the presentinvention in which aspects of the present invention may be realized;

FIG. 3 is a flow chart diagram illustrating an exemplary method forincreased efficiency managing destage tasks in a storage managementdevice having a large number of ranks, again in which aspects of thepresent invention may be realized;

FIG. 4 is an additional flow chart diagram illustrating an exemplarymethod for configuring various operations for data managementenhancement, again in which aspects of the present invention may beimplemented; and

FIG. 5 is an additional flow chart diagram of an exemplary destage TCBallocation for a rank, in accordance with the mechanisms of theillustrated embodiments.

DETAILED DESCRIPTION OF THE DRAWINGS

As mentioned previously, a storage management device (e.g., storagecontroller) in a computer storage system has a limited number of TaskControl Blocks (TCBs). In one such storage controller, the number ofsuch TCBs may be 8000 (8K). In contrast the same storage controller mayhave a larger relative number of available ranks. In the instantexample, the storage controller may have 400. An individual rank has amaximum number of destage TCBs (TCBs dedicated to executing destageoperations in the storage environment) running to destage modified datain Non Volatile Storage (NVS). In the present example, such maximumnumber of destage TCBs may be 40.

In view of such a scenario, if each rank is using its maximum number ofdestage TCBs, then the storage controller may run out of free TCBs(e.g., 40*400=16000>8K), or have very few destage TCBs to be used forother tasks in the storage controller (for example if many ranks arenear their maximum TCBs).

One possible solution is to cap the maximum destage TCBs for every rankto a lower number, such that every rank can have maximum destage TCBsand still the total will not exceed a threshold. This solution resultsin a scenario, however, where no rank may be using maximum destage TCBs,and the ranks that should use maximum destage TCBs will be running withfewer destage TCBs, and hence the throughput on the ranks, and overallsystem performance may suffer. A need exists to better manage destagetasks in a storage management device having a large number of ranks.

The mechanisms of the illustrated embodiments serve to address this needby, for example, using two counts of TCBs that may be used for destageoperations. These two counts include (a), a lower maximum destage countthat can be used for destage operations (e.g., 4K), and (b), a highermaximum destage count that can be used for destage operations (e.g.,5K).

In this way, each rank, in one exemplary embodiment, may have two“maxes” for destage TCBs: (a), a lower max, which may be defined, in oneembodiment, as the lower maximum destage TCBs for the storage managementdevice divided by the number of available ranks, and (b), an absolutemax a particular rank may use, which may be defined, again in oneembodiment, as a predetermined value (e.g., 40).

According to the mechanisms of the illustrated embodiments, a rank maybe allowed to allocate up to the lower maximum destage TCB value at anytime, but is only allowed allocations above the lower maximum destageTCB value if the total destage TCBs allocated above the lower max valuefor all available ranks is lower than a certain criterion (e.g., in oneembodiment, using the formula: Higher maximum destage TCBs for thestorage management device−Lower maximum destage TCBs for the storagemanagement device).

Turning to FIG. 1, a block diagram of one embodiment of a system 100 fordata management incorporating various aspects of the present inventionis illustrated. At least in the illustrated embodiment, system 100comprises a memory 102 coupled to a cache 104 and a processor 110 via abus 108 (e.g., a wired and/or wireless bus).

Memory 102 may be any type of memory device known in the art ordeveloped in the future. Examples of memory 102 include, but are notlimited to, an electrical connection having one or more wires, aportable computer diskette, a hard disk, a random access memory (RAM),an erasable programmable read-only memory (EPROM or Flash memory), anoptical fiber, a portable compact disc read-only memory (CD-ROM), anoptical storage device, a magnetic storage device, or any suitablecombination of the foregoing. In the various embodiments of memory 102,storage tracks are capable of being stored in memory 102. Furthermore,each of the storage tracks can be staged or destaged from/to memory 102from cache 104 when data is written to the storage tracks.

Cache 104, in one embodiment, comprises a write cache partitioned intoone or more ranks 106, where each rank 106 includes one or more storagetracks. Cache 104 may be any cache known in the art or developed in thefuture.

During operation, the storage tracks in each rank 106 are destaged tomemory 102 in a foreground destaging process after the storage trackshave been written to. That is, the foreground destage process destagesstorage tracks from the rank(s) 106 to memory 102 while a host (notshown) is actively writing to various storage tracks in the ranks 106 ofcache 104. Ideally, a particular storage track is not being destagedwhen one or more hosts desire to write to the particular storage track,which is known as a destage conflict.

In various embodiments, processor 110 comprises or has access to a cachemanagement module 112, which comprises computer-readable code that, whenexecuted by processor 110, causes processor 110 to perform datamanagement operations in accordance with aspects of the illustratedembodiments. In the various embodiments, processor 110, for each rank inthe storage management device, allocates a lower maximum count, and ahigher maximum count, of Task Control Blocks (TCBs) to be implementedfor performing a storage operation, and performs the storage operationusing up to the lower maximum count of TCBs, yet only allows those TCBsabove the lower maximum count to be allocated for performing the storageoperation satisfying at least one criterion.

In various other embodiments, processor 110 considers the at least onecriterion for one of the TCBs.

In various other embodiments, processor 110, pursuant to considering theat least one criterion, considers whether a total number of TCBspresently allocated above the lower maximum count is lower than a totalhigher maximum count of TCBs for the storage management device minus atotal lower maximum count of TCBs for the storage management device.

In various other embodiments, processor 110, pursuant to performing thestorage operation, performs a destage operation.

In various other embodiments, processor 110 establishes the lowermaximum count as a function of a total TCB for all ranks in the storagemanagement device and a total number of ranks in the storage managementdevice.

In various other embodiments, processor 110, based on an amount ofmodified data in cache, determines a number of TCBs needed for aparticular rank of the storage management device.

In various other embodiments, processor 110 determines, based on thedetermined TCBs needed for the particular rank, a number of actual TCBsto be allocated in view of the allocated lower and higher maximumamounts.

FIG. 2 is a block diagram 200 illustrating an exemplary hardwarestructure of a data storage system in which aspects of the presentinvention may be implemented. Host computers 210, 220, 225, are shown,each acting as a central processing unit for performing data processingas part of a data storage system 200. The cluster hosts/nodes (physicalor virtual devices), 210, 220, and 225 may be one or more new physicaldevices or logical devices to accomplish the purposes of the presentinvention in the data storage system 200. A Network (e.g., storagefabric) connection 260 may be a fibre channel fabric, a fibre channelpoint-to-point link, a fibre channel over ethernet fabric or point topoint link, a FICON or ESCON I/O interface. The hosts, 210, 220, and 225may be local or distributed among one or more locations and may beequipped with any type of fabric (or fabric channel) (not shown in FIG.2) or network adapter 260 to the storage controller 240, such as Fibrechannel, FICON, ESCON, Ethernet, fiber optic, wireless, or coaxialadapters. Data storage system 200 is accordingly equipped with asuitable fabric (not shown in FIG. 2) or network adapter 260 tocommunicate. Data storage system 200 is depicted in FIG. 2 comprisingstorage controllers 240 and cluster hosts 210, 220, and 225. The clusterhosts 210, 220, and 225 may include cluster nodes.

To facilitate a clearer understanding of the methods described herein,storage controller 240 is shown in FIG. 2 as a single processing unit,including a microprocessor 242, system memory 243 and nonvolatilestorage (“NVS”) 216, which will be described in more detail below. It isnoted that in some embodiments, storage controller 240 is comprised ofmultiple processing units, each with their own processor complex andsystem memory, and interconnected by a dedicated network within datastorage system 200. Moreover, given the use of the storage fabricnetwork connection 260, additional architectural configurations may beemployed by using the storage fabric 260 to connect multiple storagecontrollers 240 together with one or more cluster hosts 210, 220, and225 connected to each storage controller 240.

In some embodiments, the system memory 243 of storage controller 240includes operation software 250 and stores program instructions and datawhich the processor 242 may access for executing functions and methodsteps associated with executing the steps and methods of the presentinvention. As shown in FIG. 2, system memory 243 may also include or bein communication with a cache 245, also referred to herein as a “cachememory”, for buffering “write data” and “read data”, which respectivelyrefer to write/read requests and their associated data. In oneembodiment, cache 245 is allocated in a device external to system memory243, yet remains accessible by microprocessor 242 and may serve toprovide additional security against data loss, in addition to carryingout the operations as described herein.

In some embodiments, cache 245 may be implemented with a volatile memoryand non-volatile memory and coupled to microprocessor 242 via a localbus (not shown in FIG. 2) for enhanced performance of data storagesystem 200. The NVS 216 included in data storage controller isaccessible by microprocessor 242 and serves to provide additionalsupport for operations and execution as described in other figures. TheNVS 216, may also referred to as a “persistent” cache, or “cache memory”and is implemented with nonvolatile memory that may or may not utilizeexternal power to retain data stored therein. The NVS may be stored inand with the cache 245 for any purposes suited to accomplish theobjectives of the present invention. In some embodiments, a backup powersource (not shown in FIG. 2), such as a battery, supplies NVS 216 withsufficient power to retain the data stored therein in case of power lossto data storage system 200. In certain embodiments, the capacity of NVS216 is less than or equal to the total capacity of cache 245.

The storage controller 240 may include a data management module 112. Thedata management module 112 may incorporate internal memory (not shown)in which the destaging algorithm may store unprocessed, processed, or“semi-processed” data. The data management module 112 may work inconjunction with each and every component of the storage controller 240,the hosts 210, 220, 225, and other storage controllers 240 and hosts210, 220, and 225 that may be remotely connected via the storage fabric260. Data management module 112 may be structurally one complete moduleor may be associated and/or included with other individual modules. Datamanagement module 112 may also be located in the cache 245 or othercomponents of the storage controller 240.

The storage controller 240 includes a control switch 241 for controllinga protocol to control data transfer to or from the host computers 210,220, 225, a microprocessor 242 for controlling all the storagecontroller 240, a nonvolatile control memory 243 for storing amicroprogram (operation software) 250 for controlling the operation ofstorage controller 240, cache 245 for temporarily storing (buffering)data, and buffers 244 for assisting the cache 245 to read and writedata, and the data management module 110, in which information may beset. The multiple buffers 244 may be implemented to assist with themethods and steps as described herein.

Cache 245, in combination with data management module 112, may performvarious aspects of the present invention as will be further described,such as establishing two counts of TCBs to be allocated and appropriateallocation rules as will be further described.

One of the exemplary mechanisms of the illustrated embodiments concernsestablishing A) First threshold (any rank can allocate at any time) andb) Second threshold counts (a cap no rank can ever exceed). The deltabetween the First and Second thresholds is controlled to prevent toomany ranks from allocating too many TCBs. A rank can only allocate abovethe First threshold, if the number of TCBs (across all ranks) above theFirst threshold is under a fixed limit. For example, 200 ranks couldallocate 15 TCBs, i.e. 10 from the first threshold and 5 from the second(200*5=1K) or 50 ranks could allocate 30 TCBs (the first 10 plus asecond 20, 50*20=1K). In additional to the foregoing, any number ofcombinations known to one of ordinary skill may be implemented.

The absolute maximum number of TCBs that can ever be allocated is equalto the number of ranks times the first threshold (i.e. 400*10=4K) plusthe fixed limit (1 k) for a total of 5K.

Turning now to FIG. 3, a flow chart diagram, illustrating a generalizedmethod method 300 for cache/data management in accordance with thepresent invention, is depicted. Method 300 begins (step 302). A lowermaximum count, and a higher maximum count of TCBs are each allocated,and set aside to be implemented for performing a storage operation (step304). The storage operation is subsequently performed, where theallocated TCBs are used up to the lower maximum count, and only thosesatisfying a predetermined criterion are allowed above the lower maximumcount (step 306). The method 300 then ends (step 308).

Turning now to FIG. 4, an additional flow chart diagram of exemplaryconfiguration operations in which aspects of the illustrated embodimentsare incorporated, is depicted. Method 400 begins (step 402) by defininga lower maximum for destage TCBs for a rank (step 404). In oneembodiment, this value is the lower max of destage TCBs a rank may havefor destage operations. It may be computed, in one embodiment, asfollows:(Lower Max Destages for All Ranks in the Storage ManagementDevice/Number of Ranks)

In step 406, an absolute maximum for destage TCBs for a rank is defined.In one embodiment, this refers to the absolute maximum destage TCBs thatcan be allocated to a rank. In step 408, following, a global count ofdestage TCBs allocated above the lower maximum count is defined. Thisrefers, in one embodiment, to the global count of destage TCBs that areallocated above the lower maximum for all ranks in the storagemanagement device. In step 410, following, a global count of destageTCBs allowed over the lower maximum is defined. In one embodiment, thisrefers to the global count of destage TCBs that is allowed over thelower maximum count. The method 400 then ends (step 412).

FIG. 5, following, is an additional flow chart diagram of an exemplarydestage TCB operation for a particular rank, shown as method 500, hereagain in which aspects of the illustrated embodiments may beimplemented. Method 500 begins (step 502), with the computation of thedestage TCBs needed for the rank based on modified data in Cache (step504). If the computed destage TCB value is greater than the absolutevalue as previously defined in method 400 (step 506), then the TCBallocation is set to the absolute maximum (step 508). If not, the method500 continues to step 510, which queries if the computed destage TCBs isless than the current destage TCBs for the rank. If this is the case,then the method 500 returns to step 504 as shown. If not, the method 500continues to step 512, which queries if the computed destage TCBs neededis below the lower maximum destages for a rank.

If the answer to the query in step 512 is affirmative, then destage TCBsfor the rank are allocated (step 514). In one embodiment this may beperformed according to (allocate computed-current destage TCBs).Alternatively, method 512 moves to step 516, which queries if the globalcount of destage TCBs allocated above the lower maximum is less than theglobal destage TCBs allowed over the lower maximum value. If this is thecase, then computed-current destage TCBs are allocated for the rank(step 518), and the global count of destage TCBs allocated above thelower max is incremented by the amount of destage TCBs newly allocatedfor the rank over the lower maximum (step 520). Returning to step 516,if the query returns negative, the method 500 moves to step 520 aspreviously described. The method 500 then ends (step 522).

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims and their legal equivalents.

As will be appreciated by one of ordinary skill in the art, aspects ofthe present invention may be embodied as a system, method, or computerprogram product. Accordingly, aspects of the present invention may takethe form of an entirely hardware embodiment, an entirely softwareembodiment (including firmware, resident software, micro-code, etc.) oran embodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module,” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer-readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer-readable medium(s) may beutilized. The computer-readable medium may be a computer-readable signalmedium or a physical computer-readable storage medium. A physicalcomputer readable storage medium may be, for example, but not limitedto, an electronic, magnetic, optical, crystal, polymer, electromagnetic,infrared, or semiconductor system, apparatus, or device, or any suitablecombination of the foregoing. Examples of a physical computer-readablestorage medium include, but are not limited to, an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk,RAM, ROM, an EPROM, a Flash memory, an optical fiber, a CD-ROM, anoptical storage device, a magnetic storage device, or any suitablecombination of the foregoing. In the context of this document, acomputer-readable storage medium may be any tangible medium that cancontain, or store a program or data for use by or in connection with aninstruction execution system, apparatus, or device.

Computer code embodied on a computer-readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wired, optical fiber cable, radio frequency (RF), etc., or any suitablecombination of the foregoing. Computer code for carrying out operationsfor aspects of the present invention may be written in any staticlanguage, such as the “C” programming language or other similarprogramming language. The computer code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, or communication system, including, but notlimited to, a local area network (LAN) or a wide area network (WAN),Converged Network, or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).

Aspects of the present invention are described above with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in acomputer-readable medium that can direct a computer, other programmabledata processing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer-readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks. The computer program instructions may also beloaded onto a computer, other programmable data processing apparatus, orother devices to cause a series of operational steps to be performed onthe computer, other programmable apparatus or other devices to produce acomputer implemented process such that the instructions which execute onthe computer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the above figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

While one or more embodiments of the present invention have beenillustrated in detail, one of ordinary skill in the art will appreciatethat modifications and adaptations to those embodiments may be madewithout departing from the scope of the present invention as set forthin the following claims.

The invention claimed is:
 1. A method for data management by a processordevice in a computing storage environment, the method comprising, foreach rank in a storage management device of the computing storageenvironment: allocating a lower maximum count defined by a predeterminedlower maximum count of Task Control Blocks (TCBs) of a rank forperforming destage operations, and a higher maximum count of TCBs to beimplemented for performing a storage operation; and performing thestorage operation using up to the lower maximum count of TCBs, yet onlyallowing those TCBs above the lower maximum count to be allocated forperforming the storage operation satisfying the at least one criterion.2. The method of claim 1, further including considering the at least onecriterion for one of the TCBs.
 3. The method of claim 2, furtherincluding, pursuant to considering the at least one criterion,considering whether a total number of TCBs presently allocated above thelower maximum count is lower than a total higher maximum count of TCBsfor the storage management device minus a total lower maximum count ofTCBs for the storage management device.
 4. The method of claim 1,wherein performing the storage operation further includes performing adestage operation.
 5. The method of claim 1, further includingestablishing the lower maximum count as a function of a total TCB forall ranks in the storage management device and a total number of ranksin the storage management device.
 6. The method of claim 1, furtherincluding, based on an amount of modified data in cache, determining anumber of TCBs needed for a particular rank of the storage managementdevice.
 7. The method of claim 6, further including, based on thedetermined TCBs needed for the particular rank, determining a number ofactual TCBs to be allocated in view of the allocated lower and highermaximum amounts.
 8. A system for data management in a computing storageenvironment, comprising: a processor device; and a storage managementdevice in operable communication with the processor device, wherein theprocessor device, for each rank in the storage management device:allocates a lower maximum count defined by a predetermined lower maximumcount of Task Control Blocks (TCBs) of a rank for performing destageoperations, and a higher maximum count of TCBs to be implemented forperforming a storage operation, performs the storage operation using upto the lower maximum count of TCBs, yet only allows those TCBs above thelower maximum count to be allocated for performing the storage operationsatisfying the at least one criterion.
 9. The system of claim 8, whereinthe processor device considers the at least one criterion for one of theTCBs.
 10. The system of claim 9, wherein the processor device, pursuantto considering the at least one criterion, considers whether a totalnumber of TCBs presently allocated above the lower maximum count islower than a total higher maximum count of TCBs for the storagemanagement device minus a total lower maximum count of TCBs for thestorage management device.
 11. The system of claim 8, wherein theprocessor device, pursuant to performing the storage operation, performsa destage operation.
 12. The system of claim 8, wherein the processordevice establishes the lower maximum count as a function of a total TCBfor all ranks in the storage management device and a total number ofranks in the storage management device.
 13. The system of claim 8,wherein the processor device, determines, based on an amount of modifieddata in cache, a number of TCBs needed for a particular rank of thestorage management device.
 14. The system of claim 13, wherein theprocessor device determines, based on the determined TCBs needed for theparticular rank, a number of actual TCBs to be allocated in view of theallocated lower and higher maximum amounts.
 15. A computer programproduct for data management in a computing environment by a processordevice, the computer program product comprising a non-transitorycomputer-readable storage medium having computer-readable program codeportions stored therein, the computer-readable program code portionscomprising: an executable portion that, for each rank in the storagemanagement device: allocates a lower maximum count defined by apredetermined lower maximum count of Task Control Blocks (TCBs) of arank for performing destage operations, and a higher maximum count ofTCBs to be implemented for performing a storage operation, performs thestorage operation using up to the lower maximum count of TCBs, yet onlyallows those TCBs above the lower maximum count to be allocated forperforming the storage operation satisfying the at least one criterion.16. The computer program product of claim 15, further including anexecutable portion that considers the at least one criterion for one ofthe TCBs.
 17. The computer program product of claim 16, furtherincluding an executable portion that, pursuant to considering the atleast one criterion, considers whether a total number of TCBs presentlyallocated above the lower maximum count is lower than a total highermaximum count of TCBs for the storage management device minus a totallower maximum count of TCBs for the storage management device.
 18. Thecomputer program product of claim 15, further including an executableportion that, pursuant to performing the storage operation, performs adestage operation.
 19. The computer program product of claim 15, furtherincluding an executable portion that establishes the lower maximum countas a function of a total TCB for all ranks in the storage managementdevice and a total number of ranks in the storage management device. 20.The computer program product of claim 15, further including anexecutable portion that determines, based on an amount of modified datain cache, a number of TCBs needed for a particular rank of the storagemanagement device.
 21. The computer program product of claim 15, furtherincluding an executable portion that determines, based on the determinedTCBs needed for the particular rank, a number of actual TCBs to beallocated in view of the allocated lower and higher maximum amounts.